1. Field of the Invention
The invention pertains to the art of high frequency radiometry using comparison of the amplitude of a signal to be investigated, such as a thermal or other noise signal, to the amplitude of a locally generated reference signal. In this type of radiometer receiver, an amplitude detector is connected alternately and cyclically through a receiver to an antenna and then to a local reference signal generator. The invention more particularly relates to such a radiometric system in which cyclically operated broad band noise generation apparatus is provided for illuminating the selected target and thus augmenting the apparent temperature of the selected target with respect to its spatial background.
2. Description of the Prior Art
The comparison type of radiometer receiver has been the most widely used for the study of relatively low-level noise-like radio frequency signals, especially where the amplitudes of the noise signals to be examined are often small in comparison to the internally generated noise level within the radiometer receiver. Comparison radiometer systems achieve substantial cancellation of the receiver background noise and self-noise, permitting relatively accurate measurements of low-level radio frequency signals.
While there are other types of comparison radiometers, one popular type of radiometer particularly for use in the high frequency or microwave frequency bands is that in which an incoming signal to be investigated and a standard or calibrated reference noise signal are compared. The method employed consists essentially of the comparison of an unknown noise signal amplitude coming from the source to be examined with a known amplitude noise signal from a calibrated source, and the method can be used to measure the effective temperature of an unknown source with considerable accuracy. In such instruments, the receiver input may be switched between the antenna and the local reference signal generator at a relatively high rate, and the detected and amplified receiver output is coupled to a phase sensing detector operated in synchronism with the switching rate. The final unidirectional output signal from such a radiometer receiver is proportional to the difference between the temperature of the reference signal source and the temperature of the source viewed by the antenna, since the phase sensing detector acts automatically to subtract the receiver background or internal noise.
Passive radiometric systems, including those of the above described kind, have distinct advantages in that they are passive and therefore do not interfere with the operation of other radio equipment; they also operate with utility in most types of weather, are simple and reliable, and are relatively inexpensive. However, the performance of such prior art radiometric systems with respect to targets at relatively great ranges has been less than fully satisfactory because of the limited basic radiometric temperature contrast between such selected targets and their spatial background. Furthermore, there is a significantly increasing dilution effect at large target distances because of the constant angular width receptivity pattern of antennas normally employed in radiometer systems (i.e., as distance increases, the radiometer antenna collects proportionally more spatial background noise with respect to signals received directly from the selected target).